1. Field of the Invention
The present invention relates to an image reading apparatus which reads an image of an object (which will be generically referred to as an “image” including a character, a figure, a pattern and others in this specification) in an image input portion of a copying machine, a facsimile, a scanner and others.
2. Description of the Related Art
Conventionally, a basic structure of an image reading apparatus which reads an image of an object in an image input portion of a copying machine, a facsimile, a scanner and others comprises a light source which irradiates the object such as an original document with the light, an optical system which leads the light which is emitted from the light source and reflected by the object, and an image sensor which receives the reflected light which is led by the optical system. A xenon lamp is often used as the light source, and a CCD image sensor in which CCD elements are one-dimensionally or linearly aligned are in heavy use as the image sensor. Usually, since a length of the CCD element row of the CCD image sensor is smaller than a reading width of the object, the reflected light of the object must be reduced in size by the optical system before being led to the CCD image sensor. For example, optical information corresponding to a reading width of 300 mm must be reduced in size so as to form an image on the CCD element row having a length of 70 mm. Therefore, there is used a reduction optical system which includes a lens which performs optical reduction and a mirror which reflects the reflected light so as to deflect a light path of the reflected light from a document surface (surface to be read) to the CCD image sensor.
In this image reading apparatus, when the reduction optical system, the CCD image sensor or the like is stained with dust or grime such as paper powder, quality of read-out image is disadvantageously degraded. Therefore, the reduction optical system or the CCD image sensor must be covered with a cover member and isolated from an external space. However, a slit through which the reflected light from the object, i.e. a light for reading, is transmitted must be provided to the cover member. For the stain prevention mentioned above, the slit must be narrowed to the necessary minimum level or covered with a transparent member such as a glass plate, thereby performing spatial shield.
However, when components with a large calorific value such as a lamp, a lamp lighting circuit (inverter circuit) and others as well as the reduction optical system and the CCD image sensor are arranged in a space substantially sealed by the cover member or the like, a temperature in the sealed space disadvantageously increases. Then, an increase in the temperature causes a reduction in performance or deterioration of each component such as the CCD image sensor or any other electrical components arranged in the sealed space, which can be a factor of degradation of the read-out image quality. That is, there is demanded a countermeasure for both the stain inside the unit and an increase in the temperature which can be factors of deterioration of the read-out image quality.
Further, the performance of the light source (e.g., a xenon lamp) is quickly degraded due to aging as compared with any other components. Therefore, in order to constantly obtain the excellent read-out image quality for a long time, the light source must be appropriately replaced at fixed intervals, and thus the light source has a property of consumable supplies in a kind. When the light source is arranged in a space substantially sealed by the cover member as described above, replacement of the light source which is one of periodical maintenance operations cannot be easily performed because it requires an additional operation such as removal of the cover member.
On the other hand, in the image reading apparatus, the relative positional relationship between the light source (lamp) or a reflecting plate and a reading position on the object has a great influence on the read-out image quality. Therefore, the relative positional relationship must be accurately maintained, and a reduction or irregularities in the positional accuracy due to an assembling error or aging must be suppressed. In particular, in case of the light source which must be periodically replaced, the positional accuracy must be maintained even after replacement. Furthermore, if the external light other than the reflected light from the document surface (surface to be read) enters from the slit provided to the cover member, there occurs a problem of degradation of the read-out image quality. In order to prevent this problem, narrowing the slit width to the necessary minimum level is effective. However, in order to constantly assuredly pass the reflected light on the surface to be read in such a case, the relative positional relationship between the slit, the light source and the reading position on the surface to be read must be accurately maintained.